376 



TEXTBOOK OF BOTANY 



Some varieties of apples and pears are apparently more 

 resistant to the disease than others, but their resistance 

 seems to be dependent at least in part upon climatic 

 conditions. A variety which is fairly resistant in one 

 locality sometimes proves to be very susceptible in another 

 region. 



380. Other Bacterial Diseases. — Crown gall, which causes 

 peculiar enlargements of stems and roots at the surface of 



the soU, is becoraing a serious 

 menace in regions where apples 

 are grown. It also attacks vari- 

 ous other plants, such as rasp- 

 berries, grapes, walnuts, and 

 willows. It can be eliminated 

 only by planting such nursery 

 stock as is absolutely free from 

 the disease and by destroying 

 any plant found affected. The 

 hlack rot of cabbage has entirely 

 ruined the cabbage industry in 

 many localities. The bacteria 

 enter through the air-pores and 

 water-pores of the leaves. The 

 leaves turn to a yellowish color, 

 the veins become black, and as 

 the disease spreads to other parts 

 of the plant the leaves fall off 

 and soon the plant withers and 

 dies. A rotation of crops and 

 clean seed beds have so far been the only means of 

 control, although it seems possible that in the near future 

 resistant varieties may be developed. The bacteria that 

 cause the wilts of the cucumbers and melons live in the 

 wood cells and vessels, which thus become clogged up. 

 The parts of the plant above ground therefore do not 



Fig. 2IO. — The crown gall 

 of the apple. Soft, spongy 

 swellings are formed at or just 

 below the surface of the soil. 

 This disease is likely to be found 

 on grafted nursery stock. 



